Standard library header <flat_map> (C++23)

 

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This header is part of the containers library.

Includes
<compare> (C++20)	Three-way comparison operator support
<initializer_list> (C++11)	std::initializer_list class template
Classes
flat_map (C++23)	adapts two containers to provide a collection of key-value pairs, sorted by unique keys (class template)
flat_multimap (C++23)	adapts two containers to provide a collection of key-value pairs, sorted by keys (class template)
std::uses_allocator<std::flat_map> (C++23)	specializes the std::uses_allocator type trait (class template specialization)
std::uses_allocator<std::flat_multimap> (C++23)	specializes the std::uses_allocator type trait (class template specialization)
Functions
erase_if(std::flat_map) (C++23)	erases all elements satisfying specific criteria (function template)
erase_if(std::flat_multimap) (C++23)	erases all elements satisfying specific criteria (function template)
Tags
sorted_uniquesorted_unique_t (C++23)	indicates that elements of a range are sorted and unique (tag)
sorted_equivalentsorted_equivalent_t (C++23)	indicates that elements of a range are sorted (uniqueness is not required) (tag)

Synopsis

#include <compare>
#include <initializer_list>

namespace std {
  // class template flat_map
  template<class Key,
           class T,
           class Compare         = less<Key>,
           class KeyContainer    = vector<Key>,
           class MappedContainer = vector<T>>
  class flat_map;

  struct sorted_unique_t
  {
    explicit sorted_unique_t() = default;
  };
  inline constexpr sorted_unique_t sorted_unique{};

  template<class Key,
           class T,
           class Compare,
           class KeyContainer,
           class MappedContainer,
           class Allocator>
  struct uses_allocator<flat_map<Key, T, Compare, KeyContainer, MappedContainer>,
                        Allocator>;

  // erasure for flat_map
  template<class Key,
           class T,
           class Compare,
           class KeyContainer,
           class MappedContainer,
           class Predicate>
  typename flat_map<Key, T, Compare, KeyContainer, MappedContainer>::size_type erase_if(
    flat_map<Key, T, Compare, KeyContainer, MappedContainer>& c,
    Predicate pred);

  // class template flat_multimap
  template<class Key,
           class T,
           class Compare         = less<Key>,
           class KeyContainer    = vector<Key>,
           class MappedContainer = vector<T>>
  class flat_multimap;

  struct sorted_equivalent_t
  {
    explicit sorted_equivalent_t() = default;
  };
  inline constexpr sorted_equivalent_t sorted_equivalent{};

  template<class Key,
           class T,
           class Compare,
           class KeyContainer,
           class MappedContainer,
           class Allocator>
  struct uses_allocator<flat_multimap<Key, T, Compare, KeyContainer, MappedContainer>,
                        Allocator>;

  // erasure for flat_multimap
  template<class Key,
           class T,
           class Compare,
           class KeyContainer,
           class MappedContainer,
           class Predicate>
  typename flat_multimap<Key, T, Compare, KeyContainer, MappedContainer>::size_type
  erase_if(flat_multimap<Key, T, Compare, KeyContainer, MappedContainer>& c,
           Predicate pred);
}

Class template std::flat_map

namespace std {
  template<class Key,
           class T,
           class Compare         = less<Key>,
           class KeyContainer    = vector<Key>,
           class MappedContainer = vector<T>>
  class flat_map
  {
  public:
    // types
    using key_type               = Key;
    using mapped_type            = T;
    using value_type             = pair<key_type, mapped_type>;
    using key_compare            = Compare;
    using reference              = pair<const key_type&, mapped_type&>;
    using const_reference        = pair<const key_type&, const mapped_type&>;
    using size_type              = size_t;
    using difference_type        = ptrdiff_t;
    using iterator               = /* implementation-defined */;
    using const_iterator         = /* implementation-defined */;
    using reverse_iterator       = std::reverse_iterator<iterator>;
    using const_reverse_iterator = std::reverse_iterator<const_iterator>;
    using key_container_type     = KeyContainer;
    using mapped_container_type  = MappedContainer;

    class value_compare
    {
    private:
      key_compare comp; // exposition only
      value_compare(key_compare c)
        : comp(c)
      {
      } // exposition only

    public:
      bool operator()(const_reference x, const_reference y) const
      {
        return comp(x.first, y.first);
      }
    };

    struct containers
    {
      key_container_type keys;
      mapped_container_type values;
    };

    // constructors
    flat_map()
      : flat_map(key_compare())
    {
    }

    explicit flat_map(const key_compare& comp)
      : c()
      , compare(comp)
    {
    }

    flat_map(key_container_type key_cont,
             mapped_container_type mapped_cont,
             const key_compare& comp = key_compare());

    flat_map(sorted_unique_t,
             key_container_type key_cont,
             mapped_container_type mapped_cont,
             const key_compare& comp = key_compare());

    template<class InputIter>
    flat_map(InputIter first, InputIter last, const key_compare& comp = key_compare())
      : c()
      , compare(comp)
    {
      insert(first, last);
    }

    template<class InputIter>
    flat_map(sorted_unique_t s,
             InputIter first,
             InputIter last,
             const key_compare& comp = key_compare())
      : c()
      , compare(comp)
    {
      insert(s, first, last);
    }

    template<container-compatible-range<value_type> R>
    flat_map(from_range_t fr, R&& rg)
      : flat_map(fr, std::forward<R>(rg), key_compare())
    {
    }
    template<container-compatible-range<value_type> R>
    flat_map(from_range_t, R&& rg, const key_compare& comp)
      : flat_map(comp)
    {
      insert_range(std::forward<R>(rg));
    }

    flat_map(initializer_list<value_type> il, const key_compare& comp = key_compare())
      : flat_map(il.begin(), il.end(), comp)
    {
    }

    flat_map(sorted_unique_t s,
             initializer_list<value_type> il,
             const key_compare& comp = key_compare())
      : flat_map(s, il.begin(), il.end(), comp)
    {
    }

    // constructors with allocators

    template<class Alloc>
    explicit flat_map(const Alloc& a);
    template<class Alloc>
    flat_map(const key_compare& comp, const Alloc& a);
    template<class Alloc>
    flat_map(const key_container_type& key_cont,
             const mapped_container_type& mapped_cont,
             const Alloc& a);
    template<class Alloc>
    flat_map(const key_container_type& key_cont,
             const mapped_container_type& mapped_cont,
             const key_compare& comp,
             const Alloc& a);
    template<class Alloc>
    flat_map(sorted_unique_t,
             const key_container_type& key_cont,
             const mapped_container_type& mapped_cont,
             const Alloc& a);
    template<class Alloc>
    flat_map(sorted_unique_t,
             const key_container_type& key_cont,
             const mapped_container_type& mapped_cont,
             const key_compare& comp,
             const Alloc& a);
    template<class Alloc>
    flat_map(const flat_map&, const Alloc& a);
    template<class Alloc>
    flat_map(flat_map&&, const Alloc& a);
    template<class InputIter, class Alloc>
    flat_map(InputIter first, InputIter last, const Alloc& a);
    template<class InputIter, class Alloc>
    flat_map(InputIter first, InputIter last, const key_compare& comp, const Alloc& a);
    template<class InputIter, class Alloc>
    flat_map(sorted_unique_t, InputIter first, InputIter last, const Alloc& a);
    template<class InputIter, class Alloc>
    flat_map(sorted_unique_t,
             InputIter first,
             InputIter last,
             const key_compare& comp,
             const Alloc& a);
    template<container-compatible-range<value_type> R, class Alloc>
    flat_map(from_range_t, R&& rg, const Alloc& a);
    template<container-compatible-range<value_type> R, class Alloc>
    flat_map(from_range_t, R&& rg, const key_compare& comp, const Alloc& a);
    template<class Alloc>
    flat_map(initializer_list<value_type> il, const Alloc& a);
    template<class Alloc>
    flat_map(initializer_list<value_type> il, const key_compare& comp, const Alloc& a);
    template<class Alloc>
    flat_map(sorted_unique_t, initializer_list<value_type> il, const Alloc& a);
    template<class Alloc>
    flat_map(sorted_unique_t,
             initializer_list<value_type> il,
             const key_compare& comp,
             const Alloc& a);

    flat_map& operator=(initializer_list<value_type>);

    // iterators
    iterator begin() noexcept;
    const_iterator begin() const noexcept;
    iterator end() noexcept;
    const_iterator end() const noexcept;

    reverse_iterator rbegin() noexcept;
    const_reverse_iterator rbegin() const noexcept;
    reverse_iterator rend() noexcept;
    const_reverse_iterator rend() const noexcept;

    const_iterator cbegin() const noexcept;
    const_iterator cend() const noexcept;
    const_reverse_iterator crbegin() const noexcept;
    const_reverse_iterator crend() const noexcept;

    // capacity
    bool empty() const noexcept;
    size_type size() const noexcept;
    size_type max_size() const noexcept;

    // element access
    mapped_type& operator[](const key_type& x);
    mapped_type& operator[](key_type&& x);
    template<class K>
    mapped_type& operator[](K&& x);
    mapped_type& at(const key_type& x);
    const mapped_type& at(const key_type& x) const;
    template<class K>
    mapped_type& at(const K& x);
    template<class K>
    const mapped_type& at(const K& x) const;

    // modifiers
    template<class... Args>
    pair<iterator, bool> emplace(Args&&... args);
    template<class... Args>
    iterator emplace_hint(const_iterator position, Args&&... args);

    pair<iterator, bool> insert(const value_type& x) { return emplace(x); }
    pair<iterator, bool> insert(value_type&& x) { return emplace(std::move(x)); }
    iterator insert(const_iterator position, const value_type& x)
    {
      return emplace_hint(position, x);
    }
    iterator insert(const_iterator position, value_type&& x)
    {
      return emplace_hint(position, std::move(x));
    }

    template<class P>
    pair<iterator, bool> insert(P&& x);
    template<class P>
    iterator insert(const_iterator position, P&&);
    template<class InputIter>
    void insert(InputIter first, InputIter last);
    template<class InputIter>
    void insert(sorted_unique_t, InputIter first, InputIter last);
    template<container-compatible-range<value_type> R>
    void insert_range(R&& rg);

    void insert(initializer_list<value_type> il) { insert(il.begin(), il.end()); }
    void insert(sorted_unique_t s, initializer_list<value_type> il)
    {
      insert(s, il.begin(), il.end());
    }

    containers extract() &&;
    void replace(key_container_type&& key_cont, mapped_container_type&& mapped_cont);

    template<class... Args>
    pair<iterator, bool> try_emplace(const key_type& k, Args&&... args);
    template<class... Args>
    pair<iterator, bool> try_emplace(key_type&& k, Args&&... args);
    template<class K, class... Args>
    pair<iterator, bool> try_emplace(K&& k, Args&&... args);
    template<class... Args>
    iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args);
    template<class... Args>
    iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);
    template<class K, class... Args>
    iterator try_emplace(const_iterator hint, K&& k, Args&&... args);
    template<class M>
    pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj);
    template<class M>
    pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj);
    template<class K, class M>
    pair<iterator, bool> insert_or_assign(K&& k, M&& obj);
    template<class M>
    iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj);
    template<class M>
    iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);
    template<class K, class M>
    iterator insert_or_assign(const_iterator hint, K&& k, M&& obj);

    iterator erase(iterator position);
    iterator erase(const_iterator position);
    size_type erase(const key_type& x);
    template<class K>
    size_type erase(K&& x);
    iterator erase(const_iterator first, const_iterator last);

    void swap(flat_map& y) noexcept;
    void clear() noexcept;

    // observers
    key_compare key_comp() const;
    value_compare value_comp() const;

    const key_container_type& keys() const noexcept { return c.keys; }
    const mapped_container_type& values() const noexcept { return c.values; }

    // map operations
    iterator find(const key_type& x);
    const_iterator find(const key_type& x) const;
    template<class K>
    iterator find(const K& x);
    template<class K>
    const_iterator find(const K& x) const;

    size_type count(const key_type& x) const;
    template<class K>
    size_type count(const K& x) const;

    bool contains(const key_type& x) const;
    template<class K>
    bool contains(const K& x) const;

    iterator lower_bound(const key_type& x);
    const_iterator lower_bound(const key_type& x) const;
    template<class K>
    iterator lower_bound(const K& x);
    template<class K>
    const_iterator lower_bound(const K& x) const;

    iterator upper_bound(const key_type& x);
    const_iterator upper_bound(const key_type& x) const;
    template<class K>
    iterator upper_bound(const K& x);
    template<class K>
    const_iterator upper_bound(const K& x) const;

    pair<iterator, iterator> equal_range(const key_type& x);
    pair<const_iterator, const_iterator> equal_range(const key_type& x) const;
    template<class K>
    pair<iterator, iterator> equal_range(const K& x);
    template<class K>
    pair<const_iterator, const_iterator> equal_range(const K& x) const;

    friend bool operator==(const flat_map& x, const flat_map& y);

    friend /*synth-three-way-result*/<value_type> operator<=>(const flat_map& x,
                                                              const flat_map& y);

    friend void swap(flat_map& x, flat_map& y) noexcept { x.swap(y); }

  private:
    containers c;        // exposition only
    key_compare compare; // exposition only

    struct key_equiv
    { // exposition only
      key_equiv(key_compare c)
        : comp(c)
      {
      }
      bool operator()(const_reference x, const_reference y) const
      {
        return !comp(x.first, y.first) && !comp(y.first, x.first);
      }
      key_compare comp;
    };
  };

  template<class KeyContainer,
           class MappedContainer,
           class Compare = less<typename KeyContainer::value_type>>
  flat_map(KeyContainer, MappedContainer, Compare = Compare())
    -> flat_map<typename KeyContainer::value_type,
                typename MappedContainer::value_type,
                Compare,
                KeyContainer,
                MappedContainer>;

  template<class KeyContainer, class MappedContainer, class Allocator>
  flat_map(KeyContainer, MappedContainer, Allocator)
    -> flat_map<typename KeyContainer::value_type,
                typename MappedContainer::value_type,
                less<typename KeyContainer::value_type>,
                KeyContainer,
                MappedContainer>;
  template<class KeyContainer, class MappedContainer, class Compare, class Allocator>
  flat_map(KeyContainer, MappedContainer, Compare, Allocator)
    -> flat_map<typename KeyContainer::value_type,
                typename MappedContainer::value_type,
                Compare,
                KeyContainer,
                MappedContainer>;

  template<class KeyContainer,
           class MappedContainer,
           class Compare = less<typename KeyContainer::value_type>>
  flat_map(sorted_unique_t, KeyContainer, MappedContainer, Compare = Compare())
    -> flat_map<typename KeyContainer::value_type,
                typename MappedContainer::value_type,
                Compare,
                KeyContainer,
                MappedContainer>;

  template<class KeyContainer, class MappedContainer, class Allocator>
  flat_map(sorted_unique_t, KeyContainer, MappedContainer, Allocator)
    -> flat_map<typename KeyContainer::value_type,
                typename MappedContainer::value_type,
                less<typename KeyContainer::value_type>,
                KeyContainer,
                MappedContainer>;
  template<class KeyContainer, class MappedContainer, class Compare, class Allocator>
  flat_map(sorted_unique_t, KeyContainer, MappedContainer, Compare, Allocator)
    -> flat_map<typename KeyContainer::value_type,
                typename MappedContainer::value_type,
                Compare,
                KeyContainer,
                MappedContainer>;

  template<class InputIter, class Compare = less</*iter-key-type*/<InputIter>>>
  flat_map(InputIter, InputIter, Compare = Compare())
    -> flat_map</*iter-key-type*/<InputIter>, /*iter-mapped-type*/<InputIter>, Compare>;

  template<class InputIter, class Compare = less</*iter-key-type*/<InputIter>>>
  flat_map(sorted_unique_t, InputIter, InputIter, Compare = Compare())
    -> flat_map</*iter-key-type*/<InputIter>, /*iter-mapped-type*/<InputIter>, Compare>;

  template<ranges::input_range R,
           class Compare   = less</*range-key-type*/<R>>,
           class Allocator = allocator<byte>>
  flat_map(from_range_t, R&&, Compare = Compare(), Allocator = Allocator()) -> flat_map<
    /*range-key-type*/<R>,
    /*range-mapped-type*/<R>,
    Compare,
    vector</*range-key-type*/<R>, /*alloc-rebind*/<Allocator, /*range-key-type*/<R>>>,
    vector</*range-mapped-type*/<R>,
           /*alloc-rebind*/<Allocator, /*range-mapped-type*/<R>>>>;

  template<ranges::input_range R, class Allocator>
  flat_map(from_range_t, R&&, Allocator) -> flat_map<
    /*range-key-type*/<R>,
    /*range-mapped-type*/<R>,
    less</*range-key-type*/<R>>,
    vector</*range-key-type*/<R>, /*alloc-rebind*/<Allocator, /*range-key-type*/<R>>>,
    vector</*range-mapped-type*/<R>,
           /*alloc-rebind*/<Allocator, /*range-mapped-type*/<R>>>>;

  template<class Key, class T, class Compare = less<Key>>
  flat_map(initializer_list<pair<Key, T>>, Compare = Compare())
    -> flat_map<Key, T, Compare>;

  template<class Key, class T, class Compare = less<Key>>
  flat_map(sorted_unique_t, initializer_list<pair<Key, T>>, Compare = Compare())
    -> flat_map<Key, T, Compare>;

  template<class Key,
           class T,
           class Compare,
           class KeyContainer,
           class MappedContainer,
           class Allocator>
  struct uses_allocator<flat_map<Key, T, Compare, KeyContainer, MappedContainer>,
                        Allocator>
    : bool_constant<uses_allocator_v<KeyContainer, Allocator> &&
                    uses_allocator_v<MappedContainer, Allocator>>
  {};
}

Class template std::flat_multimap

namespace std {
  template<class Key,
           class T,
           class Compare         = less<Key>,
           class KeyContainer    = vector<Key>,
           class MappedContainer = vector<T>>
  class flat_multimap
  {
  public:
    // types
    using key_type               = Key;
    using mapped_type            = T;
    using value_type             = pair<key_type, mapped_type>;
    using key_compare            = Compare;
    using reference              = pair<const key_type&, mapped_type&>;
    using const_reference        = pair<const key_type&, const mapped_type&>;
    using size_type              = size_t;
    using difference_type        = ptrdiff_t;
    using iterator               = /* implementation-defined */;
    using const_iterator         = /* implementation-defined */;
    using reverse_iterator       = std::reverse_iterator<iterator>;
    using const_reverse_iterator = std::reverse_iterator<const_iterator>;
    using key_container_type     = KeyContainer;
    using mapped_container_type  = MappedContainer;

    class value_compare
    {
    private:
      key_compare comp; // exposition only
      value_compare(key_compare c)
        : comp(c)
      {
      } // exposition only

    public:
      bool operator()(const_reference x, const_reference y) const
      {
        return comp(x.first, y.first);
      }
    };

    struct containers
    {
      key_container_type keys;
      mapped_container_type values;
    };

    // constructors
    flat_multimap()
      : flat_multimap(key_compare())
    {
    }

    explicit flat_multimap(const key_compare& comp)
      : c()
      , compare(comp)
    {
    }

    flat_multimap(key_container_type key_cont,
                  mapped_container_type mapped_cont,
                  const key_compare& comp = key_compare());

    flat_multimap(sorted_equivalent_t,
                  key_container_type key_cont,
                  mapped_container_type mapped_cont,
                  const key_compare& comp = key_compare());

    template<class InputIter>
    flat_multimap(InputIter first,
                  InputIter last,
                  const key_compare& comp = key_compare())
      : c()
      , compare(comp)
    {
      insert(first, last);
    }

    template<class InputIter>
    flat_multimap(sorted_equivalent_t s,
                  InputIter first,
                  InputIter last,
                  const key_compare& comp = key_compare())
      : c()
      , compare(comp)
    {
      insert(s, first, last);
    }

    template<container-compatible-range<value_type> R>
    flat_multimap(from_range_t fr, R&& rg)
      : flat_multimap(fr, std::forward<R>(rg), key_compare())
    {
    }
    template<container-compatible-range<value_type> R>
    flat_multimap(from_range_t, R&& rg, const key_compare& comp)
      : flat_multimap(comp)
    {
      insert_range(std::forward<R>(rg));
    }

    flat_multimap(initializer_list<value_type> il,
                  const key_compare& comp = key_compare())
      : flat_multimap(il.begin(), il.end(), comp)
    {
    }

    flat_multimap(sorted_equivalent_t s,
                  initializer_list<value_type> il,
                  const key_compare& comp = key_compare())
      : flat_multimap(s, il.begin(), il.end(), comp)
    {
    }

    // constructors with allocators

    template<class Alloc>
    explicit flat_multimap(const Alloc& a);
    template<class Alloc>
    flat_multimap(const key_compare& comp, const Alloc& a);
    template<class Alloc>
    flat_multimap(const key_container_type& key_cont,
                  const mapped_container_type& mapped_cont,
                  const Alloc& a);
    template<class Alloc>
    flat_multimap(const key_container_type& key_cont,
                  const mapped_container_type& mapped_cont,
                  const key_compare& comp,
                  const Alloc& a);
    template<class Alloc>
    flat_multimap(sorted_equivalent_t,
                  const key_container_type& key_cont,
                  const mapped_container_type& mapped_cont,
                  const Alloc& a);
    template<class Alloc>
    flat_multimap(sorted_equivalent_t,
                  const key_container_type& key_cont,
                  const mapped_container_type& mapped_cont,
                  const key_compare& comp,
                  const Alloc& a);
    template<class Alloc>
    flat_multimap(const flat_multimap&, const Alloc& a);
    template<class Alloc>
    flat_multimap(flat_multimap&&, const Alloc& a);
    template<class InputIter, class Alloc>
    flat_multimap(InputIter first, InputIter last, const Alloc& a);
    template<class InputIter, class Alloc>
    flat_multimap(InputIter first,
                  InputIter last,
                  const key_compare& comp,
                  const Alloc& a);
    template<class InputIter, class Alloc>
    flat_multimap(sorted_equivalent_t, InputIter first, InputIter last, const Alloc& a);
    template<class InputIter, class Alloc>
    flat_multimap(sorted_equivalent_t,
                  InputIter first,
                  InputIter last,
                  const key_compare& comp,
                  const Alloc& a);
    template<container-compatible-range<value_type> R, class Alloc>
    flat_multimap(from_range_t, R&& rg, const Alloc& a);
    template<container-compatible-range<value_type> R, class Alloc>
    flat_multimap(from_range_t, R&& rg, const key_compare& comp, const Alloc& a);
    template<class Alloc>
    flat_multimap(initializer_list<value_type> il, const Alloc& a);
    template<class Alloc>
    flat_multimap(initializer_list<value_type> il,
                  const key_compare& comp,
                  const Alloc& a);
    template<class Alloc>
    flat_multimap(sorted_equivalent_t, initializer_list<value_type> il, const Alloc& a);
    template<class Alloc>
    flat_multimap(sorted_equivalent_t,
                  initializer_list<value_type> il,
                  const key_compare& comp,
                  const Alloc& a);

    flat_multimap& operator=(initializer_list<value_type>);

    // iterators
    iterator begin() noexcept;
    const_iterator begin() const noexcept;
    iterator end() noexcept;
    const_iterator end() const noexcept;

    reverse_iterator rbegin() noexcept;
    const_reverse_iterator rbegin() const noexcept;
    reverse_iterator rend() noexcept;
    const_reverse_iterator rend() const noexcept;

    const_iterator cbegin() const noexcept;
    const_iterator cend() const noexcept;
    const_reverse_iterator crbegin() const noexcept;
    const_reverse_iterator crend() const noexcept;

    // capacity
    bool empty() const noexcept;
    size_type size() const noexcept;
    size_type max_size() const noexcept;

    // modifiers
    template<class... Args>
    iterator emplace(Args&&... args);
    template<class... Args>
    iterator emplace_hint(const_iterator position, Args&&... args);

    iterator insert(const value_type& x) { return emplace(x); }
    iterator insert(value_type&& x) { return emplace(std::move(x)); }
    iterator insert(const_iterator position, const value_type& x)
    {
      return emplace_hint(position, x);
    }
    iterator insert(const_iterator position, value_type&& x)
    {
      return emplace_hint(position, std::move(x));
    }

    template<class P>
    iterator insert(P&& x);
    template<class P>
    iterator insert(const_iterator position, P&&);
    template<class InputIter>
    void insert(InputIter first, InputIter last);
    template<class InputIter>
    void insert(sorted_equivalent_t, InputIter first, InputIter last);
    template<container-compatible-range<value_type> R>
    void insert_range(R&& rg);

    void insert(initializer_list<value_type> il) { insert(il.begin(), il.end()); }
    void insert(sorted_equivalent_t s, initializer_list<value_type> il)
    {
      insert(s, il.begin(), il.end());
    }

    containers extract() &&;
    void replace(key_container_type&& key_cont, mapped_container_type&& mapped_cont);

    iterator erase(iterator position);
    iterator erase(const_iterator position);
    size_type erase(const key_type& x);
    template<class K>
    size_type erase(K&& x);
    iterator erase(const_iterator first, const_iterator last);

    void swap(flat_multimap&) noexcept;
    void clear() noexcept;

    // observers
    key_compare key_comp() const;
    value_compare value_comp() const;

    const key_container_type& keys() const noexcept { return c.keys; }
    const mapped_container_type& values() const noexcept { return c.values; }

    // map operations
    iterator find(const key_type& x);
    const_iterator find(const key_type& x) const;
    template<class K>
    iterator find(const K& x);
    template<class K>
    const_iterator find(const K& x) const;

    size_type count(const key_type& x) const;
    template<class K>
    size_type count(const K& x) const;

    bool contains(const key_type& x) const;
    template<class K>
    bool contains(const K& x) const;

    iterator lower_bound(const key_type& x);
    const_iterator lower_bound(const key_type& x) const;
    template<class K>
    iterator lower_bound(const K& x);
    template<class K>
    const_iterator lower_bound(const K& x) const;

    iterator upper_bound(const key_type& x);
    const_iterator upper_bound(const key_type& x) const;
    template<class K>
    iterator upper_bound(const K& x);
    template<class K>
    const_iterator upper_bound(const K& x) const;

    pair<iterator, iterator> equal_range(const key_type& x);
    pair<const_iterator, const_iterator> equal_range(const key_type& x) const;
    template<class K>
    pair<iterator, iterator> equal_range(const K& x);
    template<class K>
    pair<const_iterator, const_iterator> equal_range(const K& x) const;

    friend bool operator==(const flat_multimap& x, const flat_multimap& y);

    friend /*synth-three-way-result*/<value_type> operator<=>(const flat_multimap& x,
                                                              const flat_multimap& y);

    friend void swap(flat_multimap& x, flat_multimap& y) noexcept { x.swap(y); }

  private:
    containers c;        // exposition only
    key_compare compare; // exposition only
  };

  template<class KeyContainer,
           class MappedContainer,
           class Compare = less<typename KeyContainer::value_type>>
  flat_multimap(KeyContainer, MappedContainer, Compare = Compare())
    -> flat_multimap<typename KeyContainer::value_type,
                     typename MappedContainer::value_type,
                     Compare,
                     KeyContainer,
                     MappedContainer>;

  template<class KeyContainer, class MappedContainer, class Allocator>
  flat_multimap(KeyContainer, MappedContainer, Allocator)
    -> flat_multimap<typename KeyContainer::value_type,
                     typename MappedContainer::value_type,
                     less<typename KeyContainer::value_type>,
                     KeyContainer,
                     MappedContainer>;
  template<class KeyContainer, class MappedContainer, class Compare, class Allocator>
  flat_multimap(KeyContainer, MappedContainer, Compare, Allocator)
    -> flat_multimap<typename KeyContainer::value_type,
                     typename MappedContainer::value_type,
                     Compare,
                     KeyContainer,
                     MappedContainer>;

  template<class KeyContainer,
           class MappedContainer,
           class Compare = less<typename KeyContainer::value_type>>
  flat_multimap(sorted_equivalent_t, KeyContainer, MappedContainer, Compare = Compare())
    -> flat_multimap<typename KeyContainer::value_type,
                     typename MappedContainer::value_type,
                     Compare,
                     KeyContainer,
                     MappedContainer>;

  template<class KeyContainer, class MappedContainer, class Allocator>
  flat_multimap(sorted_equivalent_t, KeyContainer, MappedContainer, Allocator)
    -> flat_multimap<typename KeyContainer::value_type,
                     typename MappedContainer::value_type,
                     less<typename KeyContainer::value_type>,
                     KeyContainer,
                     MappedContainer>;
  template<class KeyContainer, class MappedContainer, class Compare, class Allocator>
  flat_multimap(sorted_equivalent_t, KeyContainer, MappedContainer, Compare, Allocator)
    -> flat_multimap<typename KeyContainer::value_type,
                     typename MappedContainer::value_type,
                     Compare,
                     KeyContainer,
                     MappedContainer>;

  template<class InputIter, class Compare = less</*iter-key-type*/<InputIter>>>
  flat_multimap(InputIter, InputIter, Compare = Compare())
    -> flat_multimap</*iter-key-type*/<InputIter>,
                     /*iter-mapped-type*/<InputIter>,
                     Compare>;

  template<class InputIter, class Compare = less</*iter-key-type*/<InputIter>>>
  flat_multimap(sorted_equivalent_t, InputIter, InputIter, Compare = Compare())
    -> flat_multimap</*iter-key-type*/<InputIter>,
                     /*iter-mapped-type*/<InputIter>,
                     Compare>;

  template<ranges::input_range R,
           class Compare   = less</*range-key-type*/<R>>,
           class Allocator = allocator<byte>>
  flat_multimap(from_range_t, R&&, Compare = Compare(), Allocator = Allocator())
    -> flat_multimap<
      /*range-key-type*/<R>,
      /*range-mapped-type*/<R>,
      Compare,
      vector</*range-key-type*/<R>, /*alloc-rebind*/<Allocator, /*range-key-type*/<R>>>,
      vector</*range-mapped-type*/<R>,
             /*alloc-rebind*/<Allocator, /*range-mapped-type*/<R>>>>;

  template<ranges::input_range R, class Allocator>
  flat_multimap(from_range_t, R&&, Allocator) -> flat_multimap<
    /*range-key-type*/<R>,
    /*range-mapped-type*/<R>,
    less</*range-key-type*/<R>>,
    vector</*range-key-type*/<R>, /*alloc-rebind*/<Allocator, /*range-key-type*/<R>>>,
    vector</*range-mapped-type*/<R>,
           /*alloc-rebind*/<Allocator, /*range-mapped-type*/<R>>>>;

  template<class Key, class T, class Compare = less<Key>>
  flat_multimap(initializer_list<pair<Key, T>>, Compare = Compare())
    -> flat_multimap<Key, T, Compare>;

  template<class Key, class T, class Compare = less<Key>>
  flat_multimap(sorted_equivalent_t, initializer_list<pair<Key, T>>, Compare = Compare())
    -> flat_multimap<Key, T, Compare>;

  template<class Key,
           class T,
           class Compare,
           class KeyContainer,
           class MappedContainer,
           class Allocator>
  struct uses_allocator<flat_multimap<Key, T, Compare, KeyContainer, MappedContainer>,
                        Allocator>
    : bool_constant<uses_allocator_v<KeyContainer, Allocator> &&
                    uses_allocator_v<MappedContainer, Allocator>>
  {};
}

References

• C++23 standard (ISO/IEC 14882:2024):

• 24.6.4 Header <flat_map> synopsis [flat.map.syn]

• 24.6.9.2 Definition [flat.map.defn]

• 24.6.10.2 Definition [flat.multimap.defn]